The use of chromium based catalysts for the gas phase production of polyethylene is very well established commercial technology. Chromium based catalysts most typically comprise a silyl chromate catalyst or a chromium oxide catalyst on an inert silica support. Such catalysts are introduced into a polymerization reactor either as a dry particulate material or as a slurry in which the solid catalyst particles are suspended in a liquid hydrocarbon diluent, such as for example mineral oil. Although both methods lead to effective polymerization catalysts, the use of a slurry feed system has been found to improve reactor operability. For example, U.S. Pat. No. 7,202,313 demonstrates that feeding a mineral oil slurry of a chromium catalyst to a gas phase reactor reduces the amount of reactor fouling associated with static build up.
U.S. Pat. No. 7,528,090 teaches that an “inert material” can be added to a support in order to modify polymerization kinetics of a supported polymerization catalyst. The inert material is added in amounts that are less than the pore volume of the support material. The preferred polymerization catalyst is a “metallocene” catalyst. It is further preferred that the inert material be added to the support prior to addition of the polymerization catalyst. The patent contemplates but does not exemplify the use of hydrocarbon oil as the inert material; preferred inert materials are polyolefin waxes such as poly-1-butene. Finally, although virtually all polymerization catalysts are contemplated, including Ziegler-Natta and chromium catalysts, the patent is silent as to the effects of mineral oil treatment on a supported chromium catalyst.
We have now found that supported chromium catalysts benefit from the addition of mineral oil in amounts which do not form a slurry or suspension of the supported catalyst in the mineral oil. Such a catalyst can be fed to a reactor using traditional dry catalyst feed equipment and produces polyethylene in a gas phase reactor with low associated fouling. This circumvents the need for capital investment into slurry feed equipment and avoids contaminating the polymer products with significant amounts of mineral oil (which is extractable with hexanes). In addition, slurry feed equipment is sometimes difficult to operate in a consistent manner due to fluctuations in catalyst component concentrations. We note that the performance of Ziegler-Natta catalyst compositions have also benefited from treatment with small amounts of mineral oil (i.e. 5 to 25% by weight of the catalyst) as taught in U.S. Pat. No. 4,548,915.